From the game's encyclopedia
Siege Weapons are Men-at-Arms that focus entirely on Sieges.
They do not have an impact on Battles, but will make a Siege complete much faster by increasing the Siege Progress.
However, Siege Weapons are only effective at besieging Holdings up to a certain Fort Level.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Siege Weapon is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Siege Weapons are Men-at-Arms that focus entirely on Sieges." The game also links it as Siege Weapons. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.